This invention relates in general to compressors and in particular to a new and useful device for controlling a turbo compressor to prevent surge.
Surging is an instable condition of a turbo-compressor, in which the conveying medium flows by jerks and jolts or periodically from the pressure side to the suction side. A performance graph based on performance characteristics of the turbocompressor such as discharge pressure and volume rate of flow of the compressor may be plotted to define a surge limit line. The surge limit line separates the stable area from the instable area. For surge limit control of the turbocompressor a blow-off line is defined in the performance graph. The blow-off line is spaced at a safety distance from the surge limit line.
In known surge controllers the instantaneous working point of the compressor in the performance graph is registered, and if the working point approaches the blow-off line, the control opens a blow-off or by-pass valve, which is connected to the outlet line of the compressor. This lowers the discharge pressure, before the working point approaches the instable area. However, if the working point changes quickly, this control may not react fast enough. This fast working point change may result in the working point crossing the blow-off line and the working point may possibly reach the surge control line.
In order to avoid this it is provided that if the working point crosses a safety line, which can be identical to the blow-off line or lie between this line and the surge control line, an additional signal is generated, which carriers out a fast, complete or partial opening of the blow-off valve.
In the case of the installation known from the U.S. Pat. No. 4 142 838 this additional signal is combined with the output signal of the control. Here the problem arises, that this correcting signal has to be eliminated after the disturbance. This is obtained in the known installation by allowing the correction signal to fade out (die down) according to a time function. The time constant of this time function has to be adapted to the adjustment behavior of the control, because the fading out (decay) of the correcting signal is a disturbance for the closed-loop control system, which has to be corrected by the control. During this fading-out time (relaxation time) the working point of the compressor is situated behind the blow-off line (i.e. toward the surge limit line).
It is also imaginable to combine the correction signal triggering the fast opening of the blow-off valve to the controller's input signal, or to a setpoint. But this has the disadvantage that the influence of the correction signal on the blow-off valve is delayed according to the controller's time behavior and that the controller's amplification factor also determines the intensity of the correction signal. This means a restriction in the free choice of controller parameters which may be used.